1. Field of the Invention
The present invention pertains to furnaces, and more particularly to a microprocessor control system and method for providing multiple heating modes in twinned furnaces utilizing a single stage thermostat.
2. Description of the Prior Art
Presently, many furnace controls include a two-stage thermostat for providing dual rate heating, wherein the first stage of the thermostat operates the furnace on low heat and the second stage of the thermostat operates the furnace on high heat. For example, a typical two-stage thermostat comprises two small mercury bulb contacts on a bi-metal sensor that close and open as a function of the movement of the bi-metal sensor in response to changing room temperatures. If the present room temperature is below a desired temperature set point, for example, one degree to two degrees below the set point, then the low mercury heat bulb contact closes to provide low heat. If the room temperature continues to drop to, for example, three degrees below the desired temperature set point, then the second mercury bulb contact closes to provide a high rate of heat. When the heating load is satisfied, both of the mercury bulb contacts in the thermostat will sequentially open or deactivate, thereby terminating the heating cycle. Thereafter, when the thermostat contacts close indicating a new heating load to be satisfied, the furnace control repeats the same identical heating cycle.
There are situations, however, where it is necessary or desirable to use two furnaces twinned together in order to provide sufficient heat for a given enclosure. This happens in the instance of large residences or commercial installations where no product in the furnace model line being used provides sufficient Btu for the needs of the installation. The physical linkage of two furnaces to achieve twinning is well known in the art.
In the prior art, twinned furnaces were generally linked together functionally so that both furnaces functioned identically as to heat provision. Thus two single stage furnaces twinned together could either both run or both not run, providing a single heating mode twinned furnace. If two stage furnaces were twinned, then either both ran in high heat, both ran in low heat, or both were off. The only way a third heating mode could be achieved was by the use of a three-stage thermostat. To do this was costly, and, even so, the blowers in both furnaces could run at different speeds.
In an earlier patent, U.S. Pat. No. 4,638,942 to Ballard et al., assigned to a common assignee, an adaptive microprocessor control system and method was taught for a single furnace which overcame the disadvantages of the requirement for a two-stage thermostat for providing dual rates of heat and the existence of large swings in room temperature. In my co-pending application Ser. No. 08/090,340 filed on Jul. 12, 1993, additional disadvantages were overcome for a single furnace, with a system which recovers quickly from setback such as, in particular, night-setback. That system also reduces off-cycle electrical power draw, which reduction can result in savings on the average cost of operation of the furnace. It is desirable to provide the latter benefits, when twinned furnaces are used, as well as allowing for the provision of multiple heating modes using a single stage thermostat, and also avoiding large swings in room temperature.